Window Clamp

ABSTRACT

A wire bonding machine window clamp assembly. The assembly includes a support plate adapted to support a leadframe strip. The assembly also includes a frame structure defining a central clamp opening adapted to expose a portion of the leadframe strip. The frame structure includes at least one elongate frame member having a first surface portion adapted to engage a top surface of the leadframe strip and a second surface portion adapted to engage upper surfaces of integrated circuit (“IC”) component stacks mounted on the leadframe strip.

CROSS REFERENCE TO RELATED APPLICATION

This Divisional Application claims priority to and the benefit of U.S.patent application Ser. No. 14/663,978 (TI-75308), filed on Mar. 20,2015, the entirety of which is incorporated herein by reference.

BACKGROUND

Semiconductor devices, to be useful, must be electrically connected toone another or to other electronic devices. Leadframes made fromconductive metal such as copper, silver or gold are often used toelectrically connect a semiconductor device to other electronic devices.One popular and flexible method of connecting semiconductor devices toleadframes is wire bonding. Bond wires usually consist of aluminum,copper or gold. Bond wire diameters typically range from about 15 μm toseveral hundred micrometers in high-power applications, There are twobasic types of wire bonding—ball bonding and wedge bonding.

In ball bonding, a small molten ball is formed at the end of the bondwire by application of a high voltage charge through a tool known as acapillary that holds and dispenses wire. The molten ball is placed onthe electrical contact surface of a chip. The contact surface is usuallycopper or aluminum. A combination of heat, pressure and ultrasonicenergy is then applied, which creates a weld between the ball and thecontact surface. The ball bond is sometimes referred to as the firstbond because it is usually the first bond made in wire bonding of an ICchip/die to a leadframe.

In a die leadframe interconnection, the type of wire bond that isgenerally used to connect the second end of the bond wire to theleadframe is called a wedge bond or sometimes a second bond. It isformed by crushing the end of the bond wire between the leadframe orother metal surface and the tip of the capillary tool.

The quality of wire bonds formed on a leadframe is dependent on a numberof factors including the stability of the leadframe on a support plateof the bonding machine.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view of a prior art wire bonding machinesupporting a leadframe strip.

FIG. 2 is a cross-sectional elevation view of a portion of a windowclamp of the prior art wire bonding machine of FIG. 1 engaged with aleadframe strip.

FIG. 3 is detail cross cross-sectional elevation view of an end portionof a leadframe and an associated component stack on which wire bondingis to be performed with the leadframe engaged by a prior art windowclamp.

FIG. 4 is a top isometric view of an example embodiment of a windowclamp of a wire bonding machine.

FIG. 5 is detail cross cross-sectional elevation view showing aleadframe portion of leadframe strip and an associated Quad FlatNo-lead) (“QFN’) component stack mounted thereon are engaged by thewindow clamp of FIG. 4.

FIG. 6 is a flow chart of a method of clamping a leadframe strip instable relationship with a leadframe support plate.

DETAILED DESCRIPTION

This specification, in general, discloses a wire bonding machine windowclamp assembly. The assembly includes a support plate adapted to supporta leadframe strip. The assembly also includes a frame structure defininga central clamp opening adapted to expose a portion of the leadframestrip. The frame structure includes at least one elongate frame memberhaving a first surface portion adapted to engage a top surface of theleadframe strip and a second surface portion adapted to engage uppersurfaces of integrated circuit (“IC”) component stacks mounted on theleadframe strip.

FIG. 1 illustrates a prior art wire bonding machine 10. The wire bondingmachine 10 has clamp holding arms 12, 14. These arms 12, 14 are adaptedto engage a window clamp 16, which, in turn, engages a leadframe strip18. The leadframe strip 18 has a plurality of integrally connectedleadframe portions 18A, 18B, 18C, etc. Each component stack 18A, etc.,has an IC component stack 21, FIG. 3 (not shown in FIG. 1) mounted onit.

Leadframe strip 18 is supported on a leadframe support plate 19. Thesupport plate 19 is mounted on a heater block 20. In some wire bondingmachine embodiments 10, the leadframe strip 18 is directly supported bythe heater block 20, and there is no separate support plate 19.

FIG. 2 is a cross-sectional elevation view of a portion of the windowclamp 16 of the prior art wire bonding machine 10 engaged with aleadframe strip 18. The window clamp 16 has a frame structure 26 thatdefines a clamp central opening 28. The bottom surface 27 of the framestructure 26 engages the underlying leadframe strip 18. Window clamp 16has a first and second support flange 22, 24, FIG. 1, extendingoutwardly from opposite sides of the frame structure 26. Support flanges22, 24 are engaged by the wire bonding machine holding arms 12, 14.Referring again to FIG. 2, the holding arms 12, 14 apply a downwardforce 25 on the support flanges that is transmitted to the framestructure 26 and the underlying portions 36 of leadframe strip 18. Thepurpose of this downward force 25 is to hold the strip 18 in stationaryrelationship with the leadframe support plate 19.

As shown in FIG. 2, the support plate 19 has a number of tiny vacuumholes 32 extending through its upper surface that are adapted to beregistered with portions of the leadframe strip 18. These vacuum holes32 are in fluid communication with a vacuum manifold (not shown) andapply a suction force 34 to the bottom surface of the leadframe strip 18to prevent it from vibrating during wire bonding operations. However,sometimes heat from the heater block 20 causes the leadframe strip 18 toexpand. As a result, a portion 38 of the leadframe strip 18 that is notengaged by the window frame structure 26 buckles upwardly, as shown inFIG. 2. These buckled portions 38 of a leadframe strip 18 tend tovibrate during wire bonding operations performed on the strip 18. Suchvibration tends to produce weak/defective wire bonds. Buckling of aleadframe strip 18 also uncovers some of the vacuum holes 32 beneath thebuckled portion 38. This causes air 34 to enter the vacuum holes 32,reducing the vacuum force of the entire system. With the reduction inthe system vacuum force, other portions of the leadframe strip 18 becomedisengaged from the plate 19, resulting in even more vibration anddefective wire bonds.

As shown by FIG. 3, in the prior art bonding machine 10, stackedcomponent assemblies 21 (e.g. clip type QFN assemblies) are mounted onleadframe portions 18A, 18B, etc., of the leadframe strip 18. Theseleadframe portions and associated component stacks 23 located at theperiphery of the leadframe strip 18 tend to micro bounce. Clipleadframes 29 on the stacks, due to their cantilever structures, alsotend to vibrate and induce further vibration and bouncing in theleadframe strip 18.

Some prior art window clamps are provided with a window pain typegridwork within the frame structure. However such gridworks, like theframe structures 26, are relatively rigid and may damage the underlyingleadframe and/or associated devices, such as integrated circuit dies, ifthe leadframe buckles. The risk of damage from such gridwork type clampsis particularly high with vertically stacked integrated circuitpackages, such as QFN packages. Another window clamp assembly, such asdescribed in U.S. patent application Ser. No. 14/294,671, filed Jun. 3,2014 of Ruby Ann Maya Mello for WINDOW CLAMP includes wire strandsrather than gridworks on a window clamp frame structure.

Window clamp assemblies, such as described with reference to FIGS. 3-6below, may be used to overcome leadframe buckling, vibration anddisengagement problems.

FIG. 4 is a top plan view of a bonding machine window clamp 50 andrelated structure of a wire bonding machine 100. The wire bondingmachine 100, except for the window clamp 50, may have the same generalstructure as conventional wire bonding machines, such a machine 10 shownin FIG. 1. FIG. 5 is a cross sectional detail of a portion of the windowclamp 50, shown in FIG. 4, and a leadframe strip 40 supported thereby.As best shown in FIG. 5, a leadframe support plate 39, which may beidentical to leadframe support plate 19 described above, supports theleadframe strip 40. The structure below the support plate 39 may be thesame as that described above with reference to FIG. 2. The leadframestrip 40 includes multiple leadframe portions, e.g. leadframe portion42, that support QFN component stacks, such as stack 44 shown in FIG. 5.The components of the QFN component stack 44 may comprise a lower die41, a lower clip leadframe 43, an upper die 45, and an upper clipleadframe 47. The components may be conventionally solder bond attached.The various components of the QFN component stack 44 and the leadframeportion 42 on which the stack 44 is mounted will ultimately beelectrically connected through wire bonding performed by the wirebonding machine,

The window clamp 50 has a central opening 51 defined by a generallyrectangular frame structure 62 that has an upwardly facing secondsurface portion 66 and a downwardly facing first surface portion 68. Theframe structure 52 includes generally linear first, second, third andfourth frame structure portions 54, 56, 58 and 60. The first and secondportions 54, 56 are generally parallel and extend perpendicular to thethird and fourth portions 58, 60. A first support flange 62 extendslaterally outwardly from the first linear portion 54 and a secondsupport flange 64 extends laterally outwardly from the second linearportion 56.

Window clamp 50 is engaged with an underlying leadframe strip 40.Leadframe strip 40 is supported by a support plate 39, which has aplurality of vacuum holes 37 arranged in a grid. The vacuum holes 37 areadapted to be placed in registration with predetermined portions, e.g.,the Leadframe portions 42 of the leadframe strip 40. In some embodimentsthe support plate 39 is supported by a heater block, such as prior artheater block 20 described above. In other embodiments the leadframestrip is supported directly by a heater block. Such direct leadframesupporting heater blocks (not shown) generally have vacuum structuresimilar to that of the support plate 39.

As best shown by FIG. 5, at least one of the elongate frame members 54,56, 58, 60, and in some embodiments all of the elongate frame members,comprise a downward facing second surface portion 69 that engages ansecond surface portion 70 of component stacks 44 positioned below it. Inthe embodiment illustrated in FIG. 5 the stack second surface portion 70is an second surface portion of an upper clip leadframe 47.

The cross-sectional view of FIG. 5 represents the cross-sectionalstructure of at least one elongate frame member, e.g., 54, 56, 58, 60from end to end thereof. In the illustrated embodiment of FIG. 4, all ofthe frame members have this configuration. As shown by FIGS. 4 and 5,each elongate frame member 54, 56, 58, 60 may comprise a flat bottomsurface 68 that is adapted to engage a tip surface 69 of the leadframestrip 40. Each frame member 54, 56, 58, 60 may also comprise anintegrally formed, lip portion 61 that extends towards the center of thecentral opening 51 in the window clamp 50. The lip portion 61 has a topsurface 63 which may be continuous with the top surface 66 of the framemember. The lip portion 61 has a downwardly facing surface portion 65and a surface portion 67 connecting the top surface portions 63 anddownwardly facing surface portion 65. The downwardly facing surfaceportion 65 is adapted to engage the top surface 70 of a QFN componentstack 44. Thus, each elongate frame member may comprise a first surfaceportion 68 adapted to engage a top surface of a leadframe strip 40 and asecond surface portion 65 adapted to engage upper surfaces 70 of ICcomponent stacks 44 mounted on the leadframe strip 40.

The above described frame structure 52, in one embodiment, engages boththe peripheral portion of a leadframe strip 40 and the top surfaces ofIC component stacks mounted adjacent to this engaged peripheral portionof the leadframe strip. Such engagement firmly holds the leadframe strip40 in engagement with the window clamp support plate 39 and obviatesmicro-bouncing of the leadframe strip and defective wire bonds caused bysuch micro-bouncing.

FIG. 6 is a flow chart of a method of holding a leadframe strip instable relationship with a leadframe support plate having vacuum holesadapted to be registered with the leadframe strip. The method includes,as shown at block 201, clampingly engaging a peripheral portion of theleadframe strip. The method also includes, as shown at block 202, urgingcomponent stacks mounted adjacent to the engaged peripheral portion ofthe leadframe strip downwardly.

Although certain embodiments of a wire bonding machine window clampassembly have been described in detail herein, alternative embodimentsof a such an assembly will become obvious to those skilled in the artafter reading this disclosure. It is intended that the appended claimsbe construed broadly to cover such alternative embodiments, except aslimited by the prior art.

What is claimed is:
 1. A wire bonding machine window clamp assembly comprising: a support plate adapted to support a leadframe strip; and a frame structure defining a central clamp opening adapted to expose a portion of said leadframe strip, said frame structure comprising at least one elongate frame member having a first surface portion adapted to engage a top surface of said leadframe strip and an second surface portion adapted to engage upper surfaces of integrated circuit (“IC”) component stacks mounted on said leadframe strip.
 2. The wire bonding machine window clamp assembly of claim 1, said frame structure comprising at least two oppositely positioned elongate frame members, each having a first surface portion adapted to engage a top surface of said leadframe strip and an second surface portion adapted to engage upper surfaces of IC component stacks mounted on said leadframe strip.
 3. The wire bonding machine window clamp assembly of claim 1, said frame structure comprising four orthogonally positioned elongate frame members each having a first surface portion adapted to engage a top surface of said leadframe strip and an second surface portion adapted to engage upper surfaces of IC component stacks mounted on said leadframe strip.
 4. The wire bonding machine window clamp assembly of claim 1 wherein said second surface portion adapted to engage upper surface portions of IC component stacks is adapted to engage upper surface portions of clip leadframes.
 5. The wire bonding machine window clamp assembly of claim 1 wherein said elongate frame member first and second surface portions are parallel.
 6. The wire bonding machine window clamp assembly of claim 1 wherein said elongate frame member first and second surface portions are flat.
 7. The wire bonding machine window clamp assembly of claim 6 wherein said elongate frame member second surface portion is adapted to engage an upper surface of a plurality of IC component clips mounted on a plurality of different IC component stacks arranged in one of a single row and a single column on said leadframe strip.
 8. The wire bonding machine window clamp assembly of claim 1 wherein said at least one elongate frame member comprises a lip portion.
 9. The wire bonding machine window clamp assembly of claim 8 wherein said lip portion comprises a downwardly facing surface portion.
 10. The wire bonding machine window clamp assembly of claim 9 wherein said lip portion extends inwardly relative to said central clamp opening.
 11. The wire bonding machine window clamp assembly of claim 9 wherein said downwardly facing surface portion of said lip portion extends parallel to said first surface portion of said frame member.
 12. The wire bonding machine window clamp assembly of claim 9, wherein said lip portion is integrally formed with said elongate frame member.
 13. A wire bonding assembly comprising: a window clamp comprising: a generally rectangular frame structure having four orthogonally arranged frame structure portions, said frame structure portions having substantially coplanar bottom surfaces, each of said four orthogonally arranged frame structure portions having an second surface portion substantially coplanar with said frame structure bottom surfaces; and a plurality of clip Quad Rat No-lead Packages (“QFNs”) assemblies each comprising: a bottom leadframe; a vertical stack of components mounted on said bottom leadframe; and a clip leadframe mounted on said vertical stack; wherein said second surface portions of said frame structure portions engage a plurality of said clip leadframes of said plurality of QFNs. 